The Southern Australian crustacean species Lomis hirta (Lomisoidea: Lomisidae) is a representative of one of the three anomuran taxa which obtained their crab-like habitus independently from each other. This process, the evolutionary transformation into a crab-like form, is termed carcinization. To shed light on the morphological changes which took place during carcinization and to investigate structural dependence (coherence) between external and internal morphological characters, we studied L. hirta and representatives of its putatively most closely related taxa, Aegla cholchol (Aegloidea: Aeglidae) and Kiwa puravida (Chirostyloidea: Kiwaidae). External and internal anatomy was studied using microcomputertomography and computer-aided 3D reconstruction. A. cholchol and K. puravida belong to equally exceptional lineages: Aegla is endemic to South America and lives in freshwater habitats; Kiwa is a deep sea dweller associated to chemosynthetic bacteria found in methane seeps or hydrothermal vents. On the basis of recent cladistic analyses we reconstruct the anatomical ground pattern of the squat lobster-like last common ancestor of the three taxa and trace the morphological transformations that affected inner and outer morphology in the recent forms. Our results show, among other things, that the pleon in Lomis underwent drastic modifications in the context of carcinization, including a reduction of the muscular portion leaving more room for the hepatopancreas and gonads, and a narrowing of the pleonal ganglia which became shifted anteriorly into the cephalothorax and attached to the cephalothoracic ganglion. We interpret these anatomical changes in Lomis to have come about because of the loss of the caridoid escape reaction, which in turn was a direct consequence of the evolution of a strongly bent pleon as part of the crab-like habitus, and of a hidden lifestyle.